This invention relates to a signal processing system for processing a signal that is to be transmitted and received across a data link and then deprocessing the signal upon receipt and, in particular, to a signal processing signal that transmits the frequency information of the communications signal as a constant amplitude signal and the amplitude information as a separate frequency signal.
There is a need to provide improved signal quality for communications over radio links, in particular, at high frequencies (HF), frequencies approximately 30 MHz and below. The CCIR recommendation 455-1 dated 1970 through 1974 and entitled "Improved Transmission Systems for HF Radio Telephone Circuits", questions 131/3, describes a method and technical standard commonly referred to as "Lincompex". The Lincompex system has been in use on "long haul" point-to-point HF circuits and, in particular, those circuits which would usually tie-in to common carrier systems. The Lincompex system provides a significant improvement in signal quality over a typical HF radio link when compared to signal quality without the Lincompex signal processing. However, there are two major disadvantages with the Lincompex systems. The absolute frequency accuracy requirement is very stringent with the Lincompex system and requires that the radio equipment employ automatic frequency control (AFC) for HF single sideband links. Very accurate frequency standards must be employed on point-to-point links which do not experience doppler frequency shift effects. Either of the above two methods is a relatively expensive radio equipment feature. Secondly, selective fading which is a result of radio frequency wave propagation, is very common on the HF frequency range. This phenomena causes relatively narrow (less than several hundred hertz) bandwidth signal degradation. This type of degradation does not normally destroy intelligibility of the voice signal, but can seriously degrade a Lincompex voice signal due to loss of the narrowband FM control channel.